The present invention relates to capacitive pressure transducers. More specifically, the present invention relates to an improved electrode support for use with capacitive pressure transducers.
FIG. 1A shows a partially sectional side view of an assembled prior art capacitive pressure transducer assembly 100. FIG. 1B shows an exploded sectional side view of transducer assembly 100. For convenience of illustration, FIGS. 1A and 1B, as well as other figures in the present disclosure, are not drawn to scale. U.S. Pat. No. 4,823,603 discloses a capacitive pressure transducer assembly of the general form of transducer assembly 100. U.S. Pat. Nos. 5,020,377 and 4,785,669 also disclose capacitive pressure transducers relevant to the present disclosure.
Briefly, transducer assembly 100 includes a body that defines a first sealed interior chamber 110, and a second sealed interior chamber 112. Chambers 110 and 112 are isolated from one another by a relatively thin, flexible, conductive diaphragm 120. As will be discussed in greater detail below, diaphragm 120 is mounted so that it flexes, or deflects, in response to pressure differentials in chambers 110 and 112. Transducer assembly 100 provides a parameter that is indicative of the amount of diaphragm flexure and this parameter is therefore indirectly indicative of the differential pressure. The parameter provided by transducer assembly 100 indicative of the differential pressure is the electrical capacitance between diaphragm 120 and an electrode 130.
Transducer assembly 100 includes a P.sub.-- x cover 140 and a P.sub.-- x body 150 (as will be discussed below, the term "P.sub.-- x" refers to an unknown pressure). FIG. 2A shows a top view of P.sub.-- x body 150. P.sub.-- x body 150 has a tubular shape and defines a central interior aperture 152 (shown in FIG. 2A and indicated by lines 153 in FIG. 1B). The upper surface of P.sub.-- x body 150 is stepped and provides a shoulder 154 that extends around the perimeter of aperture 152. P.sub.-- x body 150 also includes a lower surface 156. The P.sub.-- x cover 140 is a circular metallic sheet and is provided with a pressure tube 142 that defines a central aperture 144. The P.sub.-- x cover 140 is rigidly affixed to the lower surface 156 of P.sub.-- x body 150 (e.g., by welding). Diaphragm 120 is normally a thin, circular, flexible sheet of conductive material (e.g., stainless steel). As stated above, FIGS. 1A and 1B are not drawn to scale, and diaphragm 120 is normally much thinner than illustrated in comparison to the other components of transducer assembly 100. Diaphragm 120 contacts shoulder 154 of P.sub.-- x body 150 as indicated in FIG. 1A. The outer perimeter of diaphragm 120 is normally welded to P.sub.-- x body 150 to rigidly hold the outer perimeter of diaphragm 120 to the shoulder 154 of P.sub.-- x body 150.
P.sub.-- x cover 140, P.sub.-- x body 150, and diaphragm 120 cooperate to define the interior sealed chamber 110. P.sub.-- x cover 140 defines the bottom, P.sub.-- x body 150 defines the sidewalls, and diaphragm 120 defines the top of chamber 110. Fluid in tube 142 may flow through aperture 144, and through central aperture 152 (shown in FIG. 2A) into chamber 110. So, fluid in tube 142 is in fluid communication with the lower surface of diaphragm 120.
Transducer assembly 100 also includes a P.sub.-- r body 160 and a P.sub.-- r cover 170 (as will be discussed below, the term "P.sub.-- r" refers to a reference pressure). FIG. 2B shows a top view of P.sub.-- r body 160. P.sub.-- r body 160 has a tubular shape and defines a central aperture 162 (shown in FIG. 2B and indicated by lines 263 in FIG. 1B). The upper surface of P.sub.-- r body 160 is stepped and provides a lower shoulder 164 and an upper shoulder 166. The lower shoulder 164 extends around the perimeter of aperture 162, and the upper shoulder 166 extends around the perimeter of lower shoulder 164. P.sub.-- r body 160 also includes a lower surface 168 opposite to shoulders 164, 166. The lower surface 168 of P.sub.-- r body 160 is rigidly affixed to the upper surface of the outer perimeter of diaphragm 120 (e.g., by welding). The P.sub.-- r cover 170 is a circular metallic sheet and is provided with a pressure tube 172 which defines a central aperture 174. P.sub.-- r cover 170 is rigidly affixed to P.sub.-- r body 160 (e.g., by welding) so that the outer perimeter of P.sub.-- r cover 170 is in contact with upper shoulder 166 of P.sub.-- r body 160.
P.sub.-- r cover 170, P.sub.-- r body 160, and diaphragm 120 cooperate to define the interior sealed chamber 112. Diaphragm 120 defines the bottom, P.sub.-- r body 160 defines the sidewalls, and P.sub.-- r cover 170 defines the top of chamber 112. Fluid in tube 172 may flow through aperture 174, and through central aperture 162 (shown in FIG. 2B) into chamber 112. So, fluid in tube 172 is in fluid communication with the upper surface of diaphragm 120. As will be discussed below, electrode 130 is housed in, and does not interfere with the fluid flow in, chamber 112.
Electrode 130 is normally fabricated from a non-conducting (or insulating) ceramic block and has a cylindrical shape. FIG. 2C shows a bottom view of electrode 130. The lower surface of electrode 130 is stepped and includes a central face 135 and a shoulder 136 that extends around the outer perimeter of central face 135. Electrode 130 also defines an aperture 132 (shown in FIG. 2C and indicated by lines 133 in FIG. 1B). Electrode 130 further includes a relatively thin conductor 134 that is deposited (e.g., by electroplating) onto the central face 135. Conductor 134 is explicitly shown in FIGS. 1B and 2C, and for convenience of illustration, conductor 134 is not shown in FIG. 1A. Electrode 130 is clamped between P.sub.-- r cover 170 and the lower shoulder 164 of the P.sub.-- r body 160 as shown in FIG. 1A. Aperture 132 (shown in FIG. 2C) in electrode 130 permits fluid to freely flow through electrode 130 between the upper surface of diaphragm 120 and pressure tube 172. Clamping the electrode 130 to the P.sub.-- r body 160 holds the conductor 134 in spaced relation to the diaphragm 120. Electrode 130 is normally positioned so that the space between conductor 134 and diaphragm 120 is relatively small (e.g., on the order of 0.0002 meters).
Conductor 134 and diaphragm 120 form parallel plates of a capacitor 138. As is well known, C=Ae/d, where C is the capacitance between two parallel plates, A is the common area between the plates, e is a constant based on the material between the plates (e=1 for vacuum), and d is the distance between the plates. So, the capacitance provided by capacitor 138 is a function of the distance between diaphragm 120 and conductor 134. As the diaphragm 120 flexes up and down, in response to changes in the pressure differential between chambers 110 and 112, the capacitance provided by capacitor 138 also changes. At any instant in time, the capacitance provided by capacitor 138 is indicative of the instantaneous differential pressure between chambers 110 and 112. Known electrical circuits (e.g., a "tank" circuit characterized by a resonant frequency that is a function of the capacitance provided by capacitor 138) may be used to measure the capacitance provided by capacitor 138 and to provide an electrical signal representative of the differential pressure.
Transducer assembly 100 includes an electrically conductive feedthrough 180 to permit measurement of the capacitance provided by capacitor 138. One end 182 of feedthrough 180 contacts electrode 130. Feedthrough 180 extends through an aperture in P.sub.-- r cover 170 so that the other end 184 of feedthrough 180 is external to transducer assembly 100. The aperture in P.sub.-- r cover 170 through which feedthrough 180 extends is sealed, for example by a melted glass plug 185, to maintain the pressure in chamber 112 and to electrically insulate feedthrough 180 from P.sub.-- r cover 170. Feedthrough 180 is electrically connected to conductor 134. Electrode 130 normally includes an electroplated through hole (not shown) to permit electrical connection between conductor 134 (on the bottom surface of electrode 130) and end 182 of feedthrough 180 which contacts the top surface of electrode 130. So, feedthrough 180 provides electrical connection to one plate of capacitor 138 (i.e., conductor 134). Since diaphragm 120 is welded to P.sub.-- r body 160, the P.sub.-- r body 160 provides electrical connection to the other plate of capacitor 138 (i.e., diaphragm 120). So, the capacitance provided by capacitor 138 may be measured by electrically connecting a measuring circuit (not shown) between P.sub.-- r body 160 and end 184 of feedthrough 180. In practice, the body of transducer assembly 100 is normally grounded, so the capacitance provided by capacitor 138 may be measured simply by electrically connecting the measuring circuit to end 184 of feedthrough 180.
Conductor 134 is normally disposed in a circular "ring-like" configuration on the lower surface of electrode 130 (as indicated in FIG. 2C). Further, some prior art pressure transducers include more than one conductor disposed on electrode 130 and a corresponding number of feedthroughs to electrically connect to the conductors. Such transducers provide at least two capacitors: a first capacitor formed by diaphragm 120 and one conductor on electrode 130 and a second capacitor formed by diaphragm 120 and another conductor on electrode 130. As is known, providing multiple capacitors in this fashion can be used to advantageously provide temperature compensation for the transducer.
In operation, transducer assembly 100 is normally used as an absolute pressure transducer. In this form, chamber 112 is normally first evacuated by applying a vacuum pump (not shown) to pressure tube 172. After chamber 112 has been evacuated, tube 172 is then sealed, or "pinched off" to maintain the vacuum in chamber 112. This creates a "reference" pressure in chamber 112. Although a vacuum is a convenient reference pressure, it is also known to use other pressures as the reference pressure. Since the pressure in chamber 112 is a known or reference pressure, the components used to construct chamber 112 (i.e., P.sub.-- r body 160 and P.sub.-- r cover 170) are referred to as P.sub.-- r components (i.e., "reference pressure" components). After the reference pressure has been established in chamber 112, the pressure tube 142 is then connected to a source of fluid (not shown) to permit measurement of the pressure of that fluid. Coupling the pressure tube 142 in this fashion delivers the fluid, the pressure of which is to be measured, to chamber 110 (and to the lower surface of diaphragm 120). Since the pressure in chamber 110 is unknown, or is to be measured, the components used to construct chamber 110 (i.e., P.sub.-- x cover 140 and P.sub.-- x body 150) are referred to as P.sub.-- x components (i.e., "unknown pressure" components). The center of diaphragm 120 flexes up or down in response to the differential pressure between chambers 110 and 112. Transducer assembly 100 permits measurement of the amount of flexure of the diaphragm and thereby permits measurement of the pressure in chamber 110 relative to the known pressure in chamber 112.
Transducer assembly 100 can of course also be used as a differential pressure transducer. In this form, pressure tube 142 is connected to a first source of fluid (not shown) and pressure tube 172 is connected to a second source of fluid (not shown). Transducer assembly 100 then permits measurement of the difference between the pressures of the two fluids.
One problem with transducer assembly 100 relates to the spacing between conductor 134 and diaphragm 120. In operation of transducer assembly 100, the diaphragm 120 of course flexes up and down thereby changing the spacing between diaphragm 120 and conductor 134. However, for transducer assembly 100 to provide a consistently accurate pressure reading, it is important to provide a constant nominal spacing between diaphragm 120 and conductor 134. So for a particular pressure differential, it is important to insure that the distance between diaphragm 120 and conductor 134 is always the same. The distance between diaphragm 120 and conductor 134 for a particular pressure differential between chambers 110 and 112 may be referred to as the "nominal distance". When manufacturing large numbers of transducer assemblies 100, it is important to consistently provide the same nominal distance between conductor 134 and diaphragm 120. Further, in any one unit of transducer assembly 100, it is important to insure that the nominal distance remains constant and does not vary over time.
Prior art transducer assembly 100 includes a resilient element 192 for maintaining a constant nominal distance. Resilient element 192 is squeezed between P.sub.-- r cover 170 and the top of electrode 130. The lower shoulder 164 of P.sub.-- r body 160 supports shoulder 136 of electrode 130. Since P.sub.-- r cover 170 is welded to P.sub.-- r body 160, resilient element 192 provides a spring force that pushes down on electrode 130 and holds electrode 130 in a fixed position relative to P.sub.-- r body 160. Resilient element 192 is often implemented using a "wave washer" (i.e., a metallic O-ring type washer that has been bent in one or more places in directions perpendicular to the plane of the ring). Resilient element 192 provides a relatively large spring force (e.g., on the order of one hundred pounds) so as to hold electrode 130 in a stable position.
Although transducer assembly 100 holds the electrode 130 securely, the nominal distance between conductor 134 and diaphragm 120 can vary by small amounts over time in response to, for example, mechanical or thermal shock. As those skilled in the art will appreciate, elements that are held in place by compression, such as electrode 130, can exhibit small amounts of movement (sometimes referred to as "creep") over time. This creep can sometimes change the nominal distance and thereby adversely affect the accuracy of the transducer assembly 100. Overpressure conditions can also cause unwanted movement of electrode 130. During normal operation of transducer assembly 100, diaphragm 120 will never contact electrode 130. However, large pressures in chamber 110 beyond the normal operating range of transducer assembly 100 (i.e., overpressure), can cause diaphragm 120 to contact electrode 130 and slightly compress resilient element 192. When the overpressure condition dissipates and diaphragm 120 returns to a normal operating position, resilient element 192 re-expands and reseats electrode 130. Sometimes the new position of electrode 130 will be slightly different than the original position prior to the overpressure condition. Such changes in position can cause shifts in the nominal distance and adversely affect the accuracy of the transducer assembly 100.
It is therefore an object of the present invention to provide a pressure transducer assembly with an improved mounting for the electrode.